The present invention relates generally to position detection methods and systems, and more particularly, to a method and system for detecting a position of a mobile device or transmitter, such as a cellular telephone, based on phase differences of transmitted signals of two or more frequencies received at two or more receiving sites.
Prior position detection, or navigation, systems may be generally divided into two categories: passive navigation systems and active navigation systems. In a passive navigation system, a mobile device determines its position based on signals received from transmitters positioned at known locations. In an active navigation system, the mobile device transmits signals which are received by one or more receivers positioned at known locations. The position of the mobile device is then determined based on the received signals and the known position of the receivers.
In the past, passive navigation systems have been generally favored over active navigation systems. One reason for this favoritism is that in active systems each mobile unit must transmit signals to determine its position. Since the number of mobile units in a single active system may be in the millions, these signals may overly congest the active system and may cause the system to malfunction. This problem is exacerbated in situations where the position of the mobile unit needs to be continuously determined with a high degree of accuracy. For example, an airplane needs virtually continuous position determinations due to its high velocity and, therefore, would need to transmit signals virtually continuously. Having a large number of mobile units continuously transmitting such signals could possibly overload, or congest, an active navigation system.
One prior active navigation system was espoused by the GEOSTAR corporation. In the GEOSTAR system, at least one orbiting satellite transmits signals to mobile transponders and receives replies from the mobile transponders. For calculation purposes, the GEOSTAR system assumes that the mobile transponder is on the earth's surface. By measuring the time a signal takes to travel from a satellite to a mobile transponder and back to the satellite (loop propagation delay), the mobile transponder can be determined to lie somewhere on a calculated sphere of appropriate radius. Since the intersection of the calculated sphere and the earth's surface is a circle, the GEOSTAR system thereby locates the mobile transmitter somewhere on the circle. If two satellites are employed, the mobile transponder can be deduced to also lie on a second circle. Since the intersection of these two circles is two points, the GEOSTAR system can therefore locate the mobile transponder at one of the two points.
The GEOSTAR system unfortunately exhibits some significant deficiencies. Firstly, the GEOSTAR system suffers from the problem of congestion as experienced by other prior active navigation systems. Secondly, the GEOSTAR system relies upon loop delay measurements which are typically of questionable reliability or accuracy. For example, the accuracy of loop delay measurements is deleteriously affected by timing errors in the transponder hardware which may be caused by any number of known factors, such as temperature, imprecise manufacturing tolerances and the like. In addition, GEOSTAR systems typically use wideband signal transmissions to assure accurate loop delay measurements. Unfortunately, such wideband signal transmissions occupy a large portion of available bandwidth, thus significantly contributing to the aforedescribed problem of congestion.
Capacity is an issue at the heart of cellular communications systems and satellite-based mobile communications systems. Enough capacity for all users is guaranteed by dividing the service area into a large number of small cells with the ability to re-use the limited number of available radio frequencies again in different cells which are adequately sparated. U.S. Pat. No. 5,619,503 issued to Dent on Apr. 8, 1997 describes improvements to multi-cell or multi-beam communications systems that permit higher capacity by denser frequency re-use--ultimately permitting every frequency channel to be used for a different purpose in every cell or beam. The disclosure of U.S. Pat. No. 5,619,503 is hereby incorporated by reference herein and provides the capacity improvements necessary to allow an active navigation system to succeed. Methods to obtain an initial coarse position estimate are also described and may be used in the current invention.
There is thus a need in the art for a system and method for providing active position determination of a mobile transmitter which increases the accuracy of the position determination.